1. Field of the Invention
The present invention relates to an exhaust purification system for an internal combustion engine and a desulfurization method for the same. More particularly, the present invention relates to an exhaust purification system for an internal combustion engine and a desulfurization method for the same that recovers purification performance of a denitrification catalyst by removing sulfur poisoned at the denitrification catalyst in an exhaust system.
2. Description of the Related Art
Generally, exhaust gas flowing out through an exhaust manifold from an engine is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matters (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DE-NOx catalyst) is one type of such a catalytic converter and purifies nitrogen oxide contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) are supplied to the exhaust gas, the nitrogen oxide contained in the exhaust gas is reduced in the DE-NOx catalyst through oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst is used as such a DE-NOx catalyst. The LNT catalyst absorbs the nitrogen oxide contained in the exhaust gas when the engine operates in a lean atmosphere, and releases the absorbed nitrogen oxide when the engine operates in a rich atmosphere.
However, since a material absorbing the nitrogen oxide in the denitrification catalyst is an alkaline material, the material absorbs oxides of sulfur (material to which sulfur components contained in a fuel or an engine oil are oxidized) as well as the nitrogen oxide contained in the exhaust gas. The sulfur poisoning of the denitrification catalyst deteriorates its purification efficiency. Therefore, desulfurization of the denitrification catalyst is necessary.
According to a conventional desulfurization method for an exhaust purification system, a desulfurization of the denitrification catalyst (denitrification catalyst of nitrogen oxide absorbing type) is performed after the particulate filter is regenerated. That is, the desulfurization of the denitrification catalyst is performed at hot environment caused by heat generated when the particulate filter is regenerated.
According to the conventional desulfurization method for an exhaust purification system, the regeneration of the particulate filter and the desulfurization of the denitrification catalyst cannot be performed simultaneously. The reason is that a large amount of oxygen is necessary in the regeneration of the particulate filter, but the desulfurization of the denitrification catalyst occurs at rich atmosphere lack of oxygen. Therefore, the desulfurization of the denitrification catalyst is performed after the regeneration of the particulate filter is completed. Generally, the particulate filter is regenerated for 10-20 minutes, and the denitrification catalyst is desulfurized for 15-30 minutes. Consequently, the post-injection of fuel into a combustion chamber for the regeneration and the desulfurization for both performed for 25-50 minutes. Therefore, fuel economy may remarkably deteriorate.
In addition, if the fuel is post-injected into the combustion chamber, engines torque changes. To prevent change in the engine torque, fuel economy and driving performance may be deteriorated.
Further, if the fuel is post-injected into the combustion chamber, a large amount of PM and oil dilution may occur.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.